Use a property to define Jsonlibrary

This is suggested/recommended at
python/mypy#6002

Discovered as part of matrix-org/synapse#15052

Author: David Robertson

src/canonicaljson/__init__.py

@@ -52,7 +52,9 @@ def __call__(self, *args: Any, **kwargs: Any) -> "Encoder":
 
 
 class JsonLibrary(Protocol):
-    JSONEncoder: Encoder
+    @property
+    def JSONEncoder(self) -> Encoder:
+        pass
 
 
 # Declare these in the module scope, but they get configured in